Google has been working on the Google+ social product for a couple years now, as reported by Steven Levy and many others.

So it was only inevitable that word would leak to competitors, including Facebook.

That's exactly what happened, according to an engineer and business leader with close friends at both companies.

Apparently Facebook got wind of the Google+ feature, now called Circles, that allows users to share information with only select groups of friends, rather than their entire Facebook network.

Mark Zuckerberg took a personal interest in meeting this threat from Google, and put a team on it last summer. The result: Facebook Groups, which launched in October.

It doesn't seem to have taken off -- at least not like hugely popular features like Facebook Photos -- which suggests maybe this is a solution to a problem most people don't worry about. That doesn't bode well for Circles.